This invention relates to heart pacing apparatus. More particularly, it relates to catheters used with external and implanted pacers for temporary and permanent pacing in a clinical environment.
Temporary pacing of a patient in the post-operative period following cardiac surgery is an established and effective means of treating arrhythmias or increasing cardiac output. Additionally, temporary pacing is particularly important and widely used with respect to complete heart block, especially in patients undergoing acute myocardial infarction. Such temporary pacing has been a widely used clinical procedure in this country for many years, and the technique of inserting the catheter into the patient and pacing from an external source is a common technique of cardiology. A number of instrument manufacturers produce clinically acceptable catheters, and there are several models of pacing apparatus well known to cardiologists and suitable for carrying out this technique.
There have been two basic types of catheters available and in use for such external, or temporary pacing. A first type is what is referred to as a unipolar catheter, having one lead extending substantially the length of the catheter and being electrically connected to an electrode which is positioned inside the patient's heart for transmitting the desired electrical signal thereto. This cardiac electrode is connected through the catheter lead to a first terminal of a pacing device which, in temporary pacing, is external to the patient, and which is designed to produce a desired periodic pacing signal. The second terminal of the external pacer is connected to an electrode which is generally clamped to the patient's skin around or near the point of entry of the catheter, which may be approximately at the large vein opposite the patient's right elbow. Another suitable site, such as in the femoral vein, may also be used for catheter insertion. Such electrode must be maintained in firm electrical contact with the patient, usually requiring some sort of electrically conductive paste be applied to the patient's skin, as well as the use of additional means (such as suturing an electrode beneath the skin at the site of incision) for maintaining the electrode in firm position. When the electrodes are connected to the Pacemaker, the periodic output signals from the pacing device terminals produce biopotentials in the patient's heart of a character so as to induce stimulation of the heart, i.e., so as to pace the heart.
The second type of standard catheter in common use, is what is referred to as a bipolar catheter, having both electrodes positioned near the distal end of the catheter, such that when the catheter is fully inserted into the patient's heart, both electrodes are inside the heart and in proper position to transmit the desired signal from the pacing device directly to the patient's heart. Using the bipolar catheter, the two leads of the catheter are simply connected directly to the external pacing device, or to whatever device is in clinical use.
In a co-pending application, Ser. No. 309,996, filed Nov. 28, 1972 and which issued on July 8, 1975 as U.S. Pat. No. 3,893,461, I have disclosed a novel catheter arrangement wherein a first electrode is located substantially at the distal end of the catheter, and a second electrode is located sufficiently proximal to the distal electrode that it lies outside the heart when the catheter is positioned for pacing. The distal electrode is positioned within the heart, such that contact thereof with the heart wall provides a conduction path for the pacing signals. As set forth in that co-pending application, it is advantageous that the distal electrode be negative in polarity, i.e., operate as the cathode, relative to the proximal electrode which operates as the anode, so as to achieve cathodal pacing. Hereinafter, that application shall be referred to as "my referenced co-pending application", and is incorporated herein by reference.
In any cardiac pacing system, the stimulus signal must exceed the patient's threshold in order that a response be evoked. The threshold in turn is a function of the positioning of the catheter within the heart. In permanent implant systems, care is taken to position the catheter optimally, but in temporary pacing the catheter frequently is not placed with such care. In a small percentage of temporary pacing cases, highly variable thresholds have been encountered, occasionally resulting in transient failure to pace. While this phenomenon seems to be rare with bipolar pacing, more frequent occurrences have been reported in the literature from the early unipolar pacing art.
It accordingly is a primary object of the present invention to provide unipolar pacing apparatus having an optimal design with respect to pacing threshold variability.
It is a further object to provide such apparatus while preserving the advantageous operation of unipolar pacing relative to arrhythmias, fibrillation, or similar pathologies common to bipolar pacing.